The envelope gene of SIV and HIV encodes a glycoprotein that functions to: 1) attach to the CD4 receptor, and 2) mediates fusion of the viral and cell membrane during viral entry. Attachment to CD4 is a function of the surface (SU) subunit of envelope. Fusion is governed by the transmembrane (TM) subunit, although the SU portion also affects this process. Tissue culture studies have demonstrated that the intracytoplasmic domain of the TM protein is multi-functional in that it influences cytopathology, post- translational modification of the SU subunit, and polarized release of virions. The hypothesis of this application is that the intracytoplasmic domain of TM has functional domains that govern SIV and HIV infection in the respective susceptible host. This project will analyze the previously mentioned functions by constructing SIVmac and HIV-1 molecular clones with site-specific mutations in two amphipathic alpha helices which have been implicated in cell lysis and binding to calmodulin. Selected mutants of SIVmac239 will be tested for viral load, persistence and pathogenic potential in rhesus monkeys. Based upon the outcome of these experiments, similar experiments will be performed with monkeys infected with SIV/HIV chimeras containing site-specific mutations in the TM subunit of HIV-1. Because the attenuated SIVmac1A11 has two premature stop codons in the TM subunit, the proposed studies of site-specific mutations of the related pathogenic SIVmac239 may demonstrate that this domain of env is an important determinant of pathogenesis.